The present invention relates to a method for manufacturing a resin-molded semiconductor device with external electrodes exposed on the bottom surface thereof by encapsulating the outer periphery of the lead frame thereof (in particular, the periphery of the surface thereof on which a semiconductor chip is mounted), with a molding compound.
In recent years, in order to catch up with rapidly advancing downsizing of electronic apparatuses, it has become increasingly necessary to mount semiconductor components like semiconductor chips or pellets with higher and higher density. Correspondingly, sizes and thicknesses of semiconductor devices have also been noticeably reduced.
Hereinafter, a method for manufacturing a resin-molded semiconductor device of the type suitably applicable to downsizing and reducing the thickness of semiconductor devices and the structure of a molding die usable therefor will be described.
A PCT International Application filed on Feb. 4, 1998 (PCT/JP98/00476; priority date: Feb. 10, 1997) discloses a novel method for manufacturing a resin-molded semiconductor device or a plastic package. The contents of the above-identified application are incorporated herein by reference.
The method for manufacturing a resin-molded semiconductor device disclosed in the above-identified application will be described below with reference to FIGS. 3 through 9.
First, as shown in FIG. 3, a lead frame 4, including at least an inner lead portion 1 and a die pad portion 3 for supporting a semiconductor chip 2 surrounded by the inner lead portion 1, is prepared. It is noted that the die pad portion 3 is actually supported by a support lead portion (not shown). Depressed portions (not shown, either) are formed in the support lead portion and the die pad portion 3 is up set above the plane on which the inner lead portion 1 is located. The lead frame 4 does not include any tie bar used for preventing a molding compound supplied from flowing out during resin molding. It is also noted that the die pad portion 3 has a smaller surface area than that of the semiconductor chip 2 to be bonded thereon.
Next, as shown in FIG. 4, the semiconductor chip 2 is bonded onto the die pad portion 3 of the lead frame 4 using an adhesive. This process step is called "die bonding".
Then, as shown in FIG. 5, the semiconductor chip 2, which has been bonded onto the die pad portion 3, is electrically bonded to the inner lead portion 1 via metal fine wires 5. This process step is called "wire bonding".
Subsequently, as shown in FIG. 6, a sealing sheet (or sealing tape) 6 is attached to the bottom face of the lead frame 4 in which the semiconductor chip 2 has been bonded onto the die pad portion 3. This sealing sheet 6 is attached particularly for the purposes of providing a stopper preventing the molding compound from reaching the bottom face of the inner lead portion 1 during resin molding and of preventing any resin burr from being formed on the bottom face of the inner lead portion 1. The sealing sheet 6 is attached to cover the entire bottom face of the lead frame 4. However, the sheet 6 is adhered only to the bottom face of the inner lead portion 1, not to that of the die pad portion 3 that has been up set via the depressed portions of the support lead portion.
Thereafter, as shown in FIG. 7, the lead frame 4, to which the semiconductor chip 2 has been bonded and the sealing sheet 6 has been attached, is encapsulated with a molding compound 7. In this case, the lead frame 4 is placed within a molding die (not shown) and transfer-molded. Resin molding is performed in such a manner as to make the upper half of the molding die (i.e., upper mold) press the extended portion 8 at the ends of the inner lead portion 1 of the lead frame 4 so that the molding compound 7 does not reach the bottom face of the inner lead portion 1. Also, resin molding is performed in such a manner as to press the bottom face of the sealing sheet 6 on the inner lead portion 1 to the lower half (i.e., the lower mold) of the molding die. During resin molding, heat is generated. Because of the heat, the sealing sheet 6, attached to the bottom face of the inner lead portion 1, is thermally shrunk. As a result, a part of the sheet 6 forces its way inward (i.e., pushes its way through the supplied resin) to reach a level higher than the original level by the thickness thereof. In other words, the ingrown part of the sheet 6 doubles in thickness. Consequently, steps 9 are formed in the molding compound 7 around the bottom face of the inner lead portion 1. That is to say, the bottom face of the inner lead portion 1 protrudes from the bottom face of the molding compound 7, and therefore the inner lead portion 1 can secure a desired stand-off height. Thus, the protruding inner lead portion 1 may be used as external electrodes as it is.
Then, the sealing sheet 6 attached to the bottom face of the inner lead portion 1 is peeled off, thereby completing the inner lead portion 1 protruding from the bottom face of the molding compound 7. Finally, the extended portion 8 at the ends of the inner lead portion 1 of the lead frame 4 are cut off the lead frame 4, thereby making the end faces of the inner lead portion 1 substantially flush with the side faces of the molding compound 7 and forming external electrodes 10. In this manner, a resin-molded semiconductor device such as that shown in FIG. 8 is completed.
Next, the structure of a resin molding die used in the method for manufacturing a resin-molded semiconductor device will be described with reference to FIG. 9. FIG. 9 is a cross-sectional view illustrating a resin molding die for the resin-molded semiconductor device.
As shown in FIG. 9, the molding die 11 includes an upper mold 11a and a lower mold 11b. The upper surface of the lower mold llb is flat. By utilizing these molds, the sealing sheet 6, to be adhered to the bottom face of the lead frame 4 in which the semiconductor chip 2 has been bonded onto the die pad portion 3, forces its way inward owing to heat shrinkage so as to partially cover the side faces of the inner lead portion 1. As a result, the external electrodes 10 are formed to protrude from the bottom face of the molding compound 7.
However, the present inventors found that in the method for manufacturing a resin-molded semiconductor device, if the sealing sheet is thin, then some wrinkles are formed on the sealing sheet during resin molding. This is because such a sealing sheet is likely to thermally shrink to a large degree or tends to greatly expand and sag because of the heat applied. As a result, we found, the pattern of the wrinkles may be transferred as it is to the resin exposed on the reverse surface of the resin-molded semiconductor device and some unevenness may be formed on the reverse surface.
The present inventors also found that, on the other hand, if the sealing sheet is thick, then the thermally shrunk sealing sheet forces its way inward deeper, thereby covering a larger area of the side faces of the inner lead portion to be external electrodes and forming a deep groove in the reverse surface of the resin-molded semiconductor device. As a result, we found that the contact area between the molding compound and the side faces of the inner lead portion may be decreased and the adhesion strength of the compound to the inner lead portion may be weakened.